Willow Warbler (Phylloscopus trochilus) breeds across the northern latitudes (temperate, taiga and tundra) from western Europe to eastern Siberia. All populations migrate to Africa.
Just like Northern Wheatear, and exactly opposite to Arctic Warbler and Red-flanked Bluetail, eastern populations of Willow Warbler do not migrate to Africa along the shortest diagonal route. They instead migrate westwards towards West Asia before moving south into Africa! An incredibly long journey and one that is repeated when they go back!
While it migrates through the Arabian Peninsula to Africa, regions along the Indian west coast like Kerala may be likely destinations for birds that lose their way.